Let me say that doing warm up exercise is never going to do harm. There is no reason to believe that it is a bad thing. But for most of us, it probably does not offer many advantages.
Hands up who lunges their horse and either instantly or within a lap or two, tell them to trot? Now keep your hands up if you also start trotting your horse within seconds of sitting in the saddle to ride.
People notice when a horse yawns, blinks, changes the position of its ears, or swings its tail. There are so many tiny observations to make that tell a tale about the emotional and mental state of a horse.